Small Air Transport (SAT) is emerging as the most suitable transportation means in order to allow efficient travel, in particular for commuters, on a regional range based on the use of small airports. In this framework, the project COAST (Cost Optimized Avionic SysTem), funded by the Clean Sky JU and started in the year 2016, aims to deliver key technology enablers for the affordable cockpit and avionics, including dedicated technology for Tactical Separation decision support to the pilot. This paper first reports the high-level description of the COAST proposed Tactical Separation System (TSS) and, then, describes the design advancements of the system in the framework of the COAST project. TSS is an ADS-B-based self-separation system aimed to extend traffic situational awareness and to provide the pilot with suggested manoeuvers to maintain the required separation minima. It will constitute an enabling technology for implementation of the separation responsibility delegation to the flight segment (self-separation) in the future SESAR environment, representing a step-forward in the framework of the development of Airborne Separation Assurance Systems (ASAS) for Small Aircraft. The TSS receives consolidated traffic picture (position and velocity of all tracks) from the ADS-B receiver and its own position and velocity from the GNSS receiver. Additional surveillance information, if available, can be also sent in input to the TSS, such as TCAS status, if available. Based on this overall information in input, the TSS performs its main assigned functions, i.e. Conflict Detection and Conflict Resolution. In the paper, after a description of the implemented design process, the overall TSS architecture and interfaces and its concept of operations are outlined. Based on that, an overview of each functionality implemented in the TSS is briefly reported, namely: Coarse Filtering, Conflict Detection, Severity Assignment, Conflict Resolution and overall TSS Logic. In addition, the basic features of the dedicated TSS Human-Machine Interface are also outlined. In the paper, finally, the achievements reached up to date in the COAST project for the design of this technology and related HMI are reported and the next steps are indicated.